JAR vs. WAR

JAR

JAR (Java ARchive) is a compressed file format used to distribute Java applications or libraries.

Features

• Bundles Java class files (.class), resources (images, property files, etc.), and metadata (MANIFEST.MF, etc.) into a single file, simplifying distribution and management.
• JAR defines a MANIFEST file containing the Main-Class attribute to create an application that can be executed directly.
• It can be executed via the command (java -jar) without a separate web server, requiring only the JVM and JRE.

WAR

WAR (Web Application Archive) is a file format used to package Java web applications for deployment to web servers or WAS.

Features

• WAR files have a predefined directory structure (WEB-INF, META-INF, etc.).
• Includes deployment descriptors like Servlets, JSPs, static resources, and web.xml
• Cannot be executed directly; must be deployed and run within a web container like Tomcat, WebLogic, or WebSphere
• Highly versatile for compatibility with diverse web server environments and integration into large-scale web systems

Comparison

Conclusion

JAR files offer the advantages of standalone execution and rapid deployment,
while WAR files provide compatibility with large-scale systems and a standard web application structure.
In practice, when using embedded application servers like Spring Boot, deploying a single executable JAR file is straightforward.
For traditional web systems and integration with application servers, WAR files are more suitable.

Appendix

JVM (Java Virtual Machine): Bytecode execution engine

• A virtual machine that interprets and executes Java bytecode (.class files) to run on various operating systems
• Enables Java application execution tailored to the operating system
  Handles memory management, garbage collection, thread management, security, etc.
• Thanks to the JVM, Java achieves WORA (Write Once, Run Anywhere), meaning it operates platform-independently

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JRE (Java Runtime Environment): Runtime environment package

• Provides the environment (runtime environment) to execute Java programs
• A package containing the JVM, Java standard library, and other resource files
  Non-developer users only need to run programs, so installing the JRE alone suffices
• The JRE is included in the JDK but does not contain development tools

MANIFEST file: Defines classpath and metadata

• Located in the META-INF/ folder inside JAR or WAR files; a text file describing various metadata about the archive
• Primary uses include setting the application entry point, defining the classpath, specifying version information, etc.\

Servlet

• A server-side program written in Java
• Receives and processes HTTP requests from clients, generates dynamic web pages, and returns responses
• Manages requests and responses through HttpServletRequest and HttpServletResponse objects
• When a web server forwards a request to the WAS, the WAS executes the corresponding servlet, performs the necessary logic, and sends the results to the web browser

JSP (Java Server Pages): Web page generation technology

• Technology that dynamically generates web pages on the server by embedding Java code within HTML
• Converted to a Servlet during execution, compiled and run; acts as a template for easily writing Servlets
• Facilitates the creation of complex web pages and enables database integration, session management, etc.
• Executable on WAS

WAS (Web Application Server)

• Middleware providing the execution environment for web applications
  Processes dynamic web content
• While web servers handle static resources (HTML, images, etc.), WAS handles dynamic requests like database integration, session management, and business logic processing
• Representative WAS: Tomcat, IBM WebSphere